This invention relates generally to turbine engines and, more particularly, to reducing temperature overshoot in such engines.
In a closed system, an unmeasured temperature sometimes is regulated by a related and measured temperature. The characteristics of the unmeasured temperature differ from the measured temperature as a result of differences in thermal constants, flow fields, and thermal time constants of the measured temperature medium. As a result, the unmeasured temperature may exceed a pre-defined maximum temperature or fall below a pre-defined minimum temperature.
To maintain an unmeasured temperature within a predefined range, anticipation methods can be utilized. Generally, anticipation methods attempt to reduce temperature overshoot following a rapid change in temperature. Such anticipation methods sometimes are referred to as rate-based lead-lag anticipation. An amount, or magnitude, of measured temperature anticipation is dependent upon the rate at which the measured temperature signal changes. Thermal states are not taken into account when determining an amount of anticipation. Therefore, the same anticipation is utilized for both cold and warm thermal states.
As a result, too much anticipation may be provided for a warm thermal state and too little anticipation may be provided for a cold thermal state. For a cold thermal state, the anticipation decays in a matter of seconds when the temperature overshoot can last for a much greater time (e.g. longer than one minute).
As one specific example, and in at least one known gas turbine aircraft engine, a gas temperature T41 overshoot occurs when the gas temperature T41 is controlled by a measured high pressure turbine (HPT) metal temperature T4B. The gas temperature T41 overshoot following a cold burst from idle to full power decays over a span of one minute. The T41 overshoot characteristic is caused by a changing relationship between the measured T4B metal temperature and actual T41 gas temperature. The relationship between the T41 and T4B temperatures is altered as a result of the greater cooling effectiveness of HPT blades when the engine bore is cool (heat-soak) at idle as compared to when the rotor is warm at full power. When the engine is cool, the HPT cooling air releases heat to various metals as it passes through the engine bore. The cooled air biases the T4B measurement low and allows the unmeasured T41 gas temperature to increase. Component life can be extended, and life cycle cost can be reduced, by reducing such overshoot.
Methods and apparatus for reducing temperature overshoot in an engine, such as in a gas turbine aircraft engine, are described. In an exemplary embodiment, an unmeasured temperature to be regulated in an engine is regulated by measuring a temperature in the engine wherein the measured temperature being related to the unmeasured temperature, determining a bias of the measured temperature wherein the bias being an amount estimated to be a difference between the measured temperature and the measured temperature without cooling air beat-soak (e.g. steady state), and adding the bias to the measured temperature to restore the relationship between measured and unmeasured temperature so that the unmeasured temperature may be properly regulated by the measured temperature and estimated steady state measured temperature. The bias is determined using a heat transfer model. By using a heat transfer model to determine the bias, and then adjusting the measured temperature based on the bias, temperature overshoot can at least be reduced to facilitate extending component life and reducing life cycle costs.